pc wont start

Have just tried turning my old pc on and nothing happens, have tested the ps and it's working fine, have removed motherboard from case and taken out all connections and cards, still nothing, have tried changing memory. All that happens is the cpu fan starts and the ps fans starts and thats it?
Any more suggestions as have tried everything I can think of. Could it be the cpu at fault as the motherboard seems to be swtitching on?